This invention relates to switching waveform synthesisers for producing trigger waveforms for variable frequency poly-phase static inverters.
Three-phase pulse width modulation inverter drives rely on a balanced three-phase load, usually an induction motor, to act as an integrating filer and to minimize undesirable harmonics in the line currents as much as possible. Since inverter output waveforms exhibit half-wave symmetry, even order harmonics are not present in the waveforms and since they form a balanced three-phase system, integer multiples of third harmonic line voltage frequencies are likewise absent. The positive sequence harmonic components of the inverter line voltages applied to a three-phase induction motor load are given by 6K-5, for K integer but not zero. Likewise, the negative and zero sequence harmonics are given by GK-1 and 6K-3 respectively, for K integer but not zero. Since the latter group of harmonics are all integer multiples of the third harmonic frequency in a balanced three-phase load, zero sequence harmonic amplitudes are always zero, and only positive and negative sequence harmonic amplitudes exist in the three-phase load.
In U.S. Pat. No. 3,947,736 it has been described how the effective line voltage amplitudes of a balanced three-phase load may be varied by chopping the square wave phase waveforms with a variable width chopping pulse, applied between T/6 and T/3 and also between 2/3 T and 5T/6 of the inverter output phase voltage period, as shown in FIG. 1. Each wave period, T, is assumed to be measured from the instant of transition from the lower to the higher level of the square wave phase waveform. V.sub.1, V.sub.2 and V.sub.3 represent the chopped, three-phase voltage output waveforms and V.sub.L1, V.sub.L2 and V.sub.L3 represent the resulant balanced, three-phase line voltage output waveforms. The most significant, low order, relative harmonic amplitudes of the line voltages are shown in FIG. 2 as the duration of the single chopping pulse is altered from 0 (0% chop) to T/6 (100% chop) within the above stated chopping ranges. It can be seen that low order harmonic amplitudes rapidly approach those of the fundamental as the chop increases from 0% to 100%. However if multiple, rather than single, chopping is used an improvement is obtained. This is illustrated in FIG. 3 which shows the more significant, low order, relative harmonic amplitudes of the line voltages when the single chopping pulse (duration y) is replaced by four equally spaced chops each of duration Y/4, within the above stated chopping ranges.
A detailed study of waveform harmonics, such as those shown in FIGS. 2 and 3, reveals that the predominant line voltage harmonics applied to the balanced three-phase load (other than the fundamental) occur in pairs and a negative sequence harmonic component is always balanced by a positive sequence harmonic component. If n is the number of equally spaced voltage control chops per inverter phase half-cycle which are applied between the previously stated ranges of T/6 and T/3 and also 2/3T and 5T/6 of the phase waveform period, then these predominant harmonic pairs are given by 6Kn.+-.1, for K integer but not zero. For this reason, current harmonics do not markedly affect the torque produced by a thre-0hae induction motor load, but do contribute to undesirable power losses in the motor. To reduce the amplitudes of predominant harmonic currents as much as possible in the integrating filter formed by the balanced three-phase load, it is desirable to make n as large as possible.
Practical limits to the value of n exist. One practical limit to the value of n is imposed by the maximum switching frequency of the static inverter switching devices, which will usually be silicon controlled rectifiers. Since the switching frquency of an inverter SCR switched by these waveforms is (2n+1) times the inverter fundamental frequency, a value of n suitable at low frequencies would cause the inverter SCRs to exceed their maximum switching frequency limit at higher frequencies.